Piezoelectric crystal lapping apparatus



30, 1951 J. M. WOLFSKILL 2,539,561

PIEZOELECTRIC CRYSTAL LAPPING APPARATUS Filed Dec. 13, 1946 I 2 Sheets-Sheet l Jan. 30, 1951 Filed Dec. 13, 1946 J. M. WOLFSKILL PIEZOELECTRIC CRYSTAL LAPPING APPARATUS 2 Sheets-Sheet 2 Patented Jan. 30, 1951 UNITED STATES-PATENT OFFICE.

PIEZGEISECTRIC CRYSTAL LAPPINGE APPARAZ IUS .IohnM. Wolfskill, Eric Pa. Application December 13, I946, sesame; 7f6068- This invention relatesto apparatusandmethod for grinding and lapping parts-01 pieces of mate.- rial either irregular or regular: in shape torelatively thin dimensions particularly to the; grinding and lapping of various shaped piezoelectric quartz crystalsand electrodes therefor..

An object of this invention is to provide an apparatus for lapping piezoelectric; crystals to extremely thin: dimensions and close tolerances.

Another object of this invention. is to provide an apparatus: for. lapping piezoelectric. crystals: to thicknesses. of less than ifi -ot an inch; and close tolerances.

Another object of this invention is to provide means whereby a crystal or other part or piece of material may be lapped to a thickness which is not dependent onthe thickness of any carrier, holding, or driving meanssurmunding the crystal or other part or piece of material being lapped.

Another objectof this invention is: to provide a lapping apparatus which the articles being lapped are made to movein a number of different paths across a lapping surface the movement of which is independent of the movement of the article carrier.

A further object of this invention is to provide a method of lapping quartz crystals' or similar pieces to thicknesses approaching the thickness of the abrasive particles employed It is another objectof this invention to provide videapparatus whereby parts may be lapped to;

difficult and exacting tolerances such as are rel (mired in the grinding andlapping of piezoelectric quartz crystals to: high frequencies. A further object 0t this invention is te provide 'means for lapping piezoelectric er-ystal's'and electrodes therefor or other devices to the required degree of surfacefinish:- encountered in high frequency operation;

It. is still an-otherobject of this: invention to provide a means 0&2 regulating the speed of'lapping the surtaces ofpi'ezoelectric crystals and/or electrodes therefor by controlling the weight thereof A still further object 0t this invention is to provide a means of controtlingthe shape of the lapped surface or itseontour to within extremely close tolerance by regulating the throw on the lapping-plates or regulating the weight and shape oi the plate con-tact with the piece" bein lap t or. 51 -133);

It is another object: to; provide. a. means. for lapping large quantities. of. pieces. to extremely close tolerance. without. resorting to painstaking individualized operations;

Still anothersobjeet ofv this invention is, to provide a lap. for: lapping. piezoelectric: crystals, said lap having: a. lap: plate provided: with holes; for receiving the piezoelectric crystals and weightslin said holes for applying: pressure to the crystals during" the. lapping; operation.

Still. another'objiect. of thisainventionr is to provide lapping apparatustfor: producing extremely thin and accurate crystalsgsaid.apparatus: having a lap plate provided; with holes for receiving piezoelectric crystal blanks. and. guiding said blanks during the lapping operation.

Still another object oi this invention is toprovide a lap for. lapping. piezoelectric; crystals, said lap having a lap plate provided; with holes. for receiving; the piezoelectric crystals. and: insulated weights in. said holes. for applying pressure to; the crystals during the lapping operation and for contacting said crystals for the purpose of measuring the frequency-thereof.

Other and further objectsoi this-invention. will be apparent to those skilled in. the art: to which it relates from the fiolloivingspecification, drawing and claims.

In' the. prior art of: lapping quartz crystals: or other surfaces theipiecebeingz lappedis generally handled in one. of two: ways. Either the. piece is. cemented to a flat: Work; holder by means. of paraffin or similar:wax or other pitch; either. individually or-i'n large quantities:- depending. on the size. of. the pieces. and the" entire work. holder is moved. about over another flat surface with an abrasive and vehicle between the aforesaid flat surface and surfaces being. lapped.

Another method: commonly used in the piezoelectric crystal. lapping. art is: that in: which the thin pieces. The distortion takes place when the quartz-crystal or other material iscemented to the work holder. The resulting lapped surfaces of the quartz or metal piece may be exceptionally fiat prior to the removal thereof from the work holder but this surface may have been originally distorted in the cementing operation. Upon removal of the quartz crystal or other material from the work holder after lapping distortions introduced therein in the course of the cementing operation are relieved and the quartz crystal or other material springs back to its naturally free condition so that the surface lapped onto it, either concave or convex, may or may not be regular. In addition, the thickness of the cementing medium between the pieces being lapped and the work holder may vary to such an extent that it is impossible to lap them to any specific thickness because the fastening medium varies and furthermore there is no telling what contour the v work pieces have, until after removal thereoffrom the work holder.

The other method in the prior art has the definite disadvantage in that the thickness to which the quartz crystals or other material being lapped may be lapped is the thickness of the carrier or work holder which must be of a thickness and strength to move the work pieces, that is, the quartz crystals or other material, between the laps without tearing, fracturing or buckling. Regardless of what material such carriers or work holders are made, or regardless of the pressure on the lapping surface or between laps and work piece this disadvantage remains. There appears to be a practical limit to the thickness or thinness of the work holders of about .004" or .005" where the work holders tear, fracture and/or buckle.

It is the purpose of this invention to disclose a method of lapping to extremely thin dimensions with the limits in minimum thickness fixed only by the precision to which the lapping plates are made, and to the maximum size of the abrasive grain used.

Briefly, this apparatus comprises a top lapv that is rotated over a bottom lap such that the top lap makes several rotations for each rotation of the bottom lap. The ratio of five to one or six to one has been used in this development but this can be varied over wide limits. The top lap ismoved by a central pivot arm in such a manner that the abrasive action on the bottom lap remains consistent over its entire surface. The radius of the top lap movement can be varied as-well as the center of the top lap movement to obtain uniform wear on the bottom lap. A plurality of holes are 'drilled either radially or in any configuration desired through the top lap and the work pieces .placed in these holes. The work pieces themthe bottom ends of the freely floating cylinders press on the work pieces. The bottom surfaces of these cylinders may be fiat to produce flat work; piece or crystal surfaces or they can be made convex, for instance so that most of the pressure Will be in the central area of the work piece and this will produce a concave surfaceif it is desired. For instance, to lap a quartz crystal con-.-

cave this method would be used. If it is desired to lap one face of the crystal convex, the end of the cylinder pressing against the crystal could be 'made concave in which case the pressure would be around the periphery of the crystal and this;

portion would be lapped down faster than the center area. In thin work pieces, such as encountered in piezoelectric crystals, a similar effect can be produced by hand operation in which case the finger or a pencil point can provide pressure at different points over the crystal surface as it is being hand lapped. In producing extremely flat surfaces the cylinder faces themselves must be flat so that the pressure is uniformly distributed over the crystals or Work piece surfaces. Such a flat surface on the cylinder face is easily established by lapping the entire top lap with its complement of cylinders without the work piece's i nserted. This can be carried on prior to loading of the top lap with piezoelectric crystal blanks.

Round as well as square crystals can be lapped in this apparatus. However, if a square or rectangular shaped crystal or work piece is lapped, the sideways or lateral pressure on the corners may become excessive if the square or rectangular crystals are positioned in the round holes of the top lap. If the crystal is extremely thin, the pressure may be suflicient to fracture the corners. In such instances, the top lap may be further modified either by using a square hole originally in the top lap, which is difficult to machine, or by introducing a free cylinder with a square hole, through it and in which a free floating square or rectangular weight can be placed. In order to get around the difficulty of machining a square hole through a cylinder the original round cylinder may be split into two parts or may even consist of three or four parts. If it is split the two similar cylinders can be milled in the center so as to form any desired shape internally to accommodate the square or rectangular work pieces.

Further details of this invention will be apparent to those skilled in the art to which it relates from the following specification, claims and drawing in which briefly: 1

Fig. 1 is a View of an embodiment of this paratus;

Fig. 2 is a top view of the top lap plate showing the piezoelectric crystal receiving holes;

Fig. 3 isja fragmentary sectional View taken along the line33 of Fig. 2;

Fig. 4 is a top view of a modified form of top plate with two circular rows of piezoelectric crystal receiving holes formed therein;

Fig. 5 is a view of a circular plug with a rectangular hole formed therein to be employed in the crystal receiving holes of the plates shown in Figs. 1, 2, 3 and 4 for adapting the top plates of this apparatus for rectangular crystal blanks;

* Fig. 6 is a weight of bar like configuration having a rectangular cross section to fit into the rectangular holes of the elements shown in Fig. 5 for the purpose of pressing on the rectangular crystal blanks;

i Fig. '7 is a modified form of this apparatus; Fig. 8 is a View of the top plate of the apparatus shown in Fig. 7 in which the pressure plate vex face and Fig. 13 is" a sectional view of a weight having a convex bottom for resting on the top of a crystal to produce a crystal lapped with a concave face.

Referring to Fig. -1 in detail, there is illustrated an embodiment of this apparatus employinga bottom lap plate 10 adapted to be supported and rotated by the shaft H which is mounted in bearings 12. Appropriate belt or gear drive means i3 is employed between the shaft H and the motor M for rotating the bottom plate it. The lap plate 15 is arranged to move over the top surface of the lap t and is driven by the pin It of hardened steel and eccentric I! which is attached to the lower end of the shaft 1 8-. The shaft 18 is provided with a bearing F9 and is driven by the motor 2 I through the belt or gear arrangement 2 It is obvious that one motor may be employed in place of the motors M and 2! for driving both of the shafts H and I8 through suitable belt or gear arrangements. It is however desirable to rotate the eccentric crank 1 at a speed bearing a. ratio of a prime number such as 5, 1, and I t, etc., with respect to the speed of the bottom lap to. The purpose of this being that a point on the .top lap I will describe a large number of epi-cycloidalpaths on the bottom lap 10 before that point-travels over the same course on the bottom lap Hi again. These ratios however are not necessary for the production of thin crystals although employing one of the aforesaid prime number ratios will maintain flatness or a particular contour of the bottom lap f0 over a greater period of time. Furthermore the top lap is rotated over the bottom lap H] such that the top lap makes several rotations for each rotation of the bottom lap. A ratio of 5 to 1 or 6 to 1, that is, either '5 revolutions of the top lap l5 to one revolution of the bottom lap H1 or 6 revolutions of the top lap I5 to one revolution of the bottom lap II] was employed in this apparatus. However, it will be appreciated that this ratio maybe varied over wide limits and that it is not necessary to cmploy the aforesaid '5 to 1 or '6 to 1 ratios. Also the radiusof the swing of the top lap l5 may be varied by lengthening or shortening the arm of the eccentric I! and this may be done readily by loosening the set screw Ila which retains the "eccentric H in a hole provided therefor at thelower end of the shaft l8. Thusthe length of the eccentric I'I- may be varied periodically .so that the radius of swing of the top lapv l5 is varied and produces substantially uniform wear on the bottom lap ID.

The: lap I5 is provided with a plurality of holes for receiving the plugs 22 therein. The plugs 22 function as weights for pressing the work pieces 23-, which in this case are the quartz piezoelectric crystal blanks, against the top lap H) as shown in Figv 3. The plugs 22 are fitted into the .holes so that substantially all of the weight of each individual plug is uniformly distributed upon the respective piezoelectric crystal face associated therewith. However these plugs must not be so loose that they wobble in the holes inasmuch as such a loose fit would produce distorted lapping of the piezoelectric crystal since the weight of the plugs would not be uniform y distribut d over the surfaces of the corresponding crystals.

In the embodiment shown in Figs. 1 and '2 only one circular row of piezoelectric crystal receiving holes is provided to the lap plate 1-5. However it is obvious that several circular rows of holes may be provided as shown in Fig. 4..

In Figs. 5 and 6 is illustrated a fixture consisting of a cylindrical insert made in two sections 24 and 25 of such size as to fit snugly into the piezoelectric crystal receiving holes of the laps l5 of -l-5afor adapting these holes for receivingrecta-ng-ular piezoelectric crystal wafers or blanks. The holes formed in the laps l5 and 15a are of circular configuration as shown for the purpose of receiving circular piezoelectric crystal wafers. However when it is desired to lap rectangular crystal wafers the insert shown in Fig. 5 having an outside circular configuration and a rectangular hole the-rethroughis snugly fitted into each of the crystal receiving holes on the laps 15 and 15a and the rectangular bar 26 shown in- Fig. 6- is-placed intothe axial hole of the insert 24, 25 to function as a weight distributed uniformly on the top of the crystal blank being lapped. The insert is made 'in two sections 24 and 25 inasmuch as this facilitates the manufacture thereof since it is quitedifficul-t to out arectangular hole axially through an elongated rod.

It is of course obviousthat the lap- 15 maybe employed for lapping both circular crystal wafers and rectangular crystal wafers in the different crystal receiving apertures thereof at the same time. Furthermore crystal wafers of different thicknesses may be lapped in different holes of the laps I 5 and l5a simultaneously.

Also the weights 22 may have the cylindrical sides. thereof coated with insulating material and functionv as. electrodes. on the topfaces of the crystal blanks 23 being lapped so that the frequencies of these crystal blanks may be checked on suitable frequency measuring Lapparatus .50 -(shown in Fig. 10) simply by connecting the :bottom lap I10 to one side of the inputof the frequency measuring apparatus by means of a suitablev brush or Wiping contact or through the bearing of the lap l0 and the other side of the input of this apparatus may be connected selectively to the different weights 22. Likewise the inserts shown in Fig. 5 may be made of insulating material or the elongated sides of the weight 26 may be coated with insulation instead so that the weight 26 shown in Fig. 6 may be used as an electrode on the top surface of the rectangular crystal wafer being lapped. In this way, the frequencies of the crystal wafers may be checked without removing the crystal wafers from the lap. If desired the bottom portion of each of the weights 22 and 26 may be made of insulation material :51 and a small button electrode. 52 supported on this insulation material for the purpose of making frequency measurements of the crystal wafer 23, as shown in Fig. 11. In this case the button electrodes each must of course be connected to a suitably insulated segment 54 on the top of the weights '22 and 26 by a piece of insulated wire 55. l

It is of course necessary to have the bottom of the weights 2'2 and 2-5 flat if flat piezoelectric crystal blanks are to be produced in this lapping apparatus and likewise the electrode 52 and insulation '51 must have the bottom surface thereof lapped flat for producing flat crystals. 'Where it is desired to produce crystal blanks 23 each having a convex face or faces; the

weights 22a, each having a concave face as shown in Fig. 12, are employed in rotatable laps l5 and lea. On the other hand, if it is desired to produce crystal blanks, each having a concave face or faces, then the weight member 22b,

having a bottom surface that is convex as shown in Fig. 13, is employed in each of the holes of the laps l5 or l5a.

In Figs. '7, 3 and 9 as illustrated a modified form of this invention in which an annular pressure plate 2'! is attached to the top of the lap [5 by suitable studs 28 and wing nuts 29. This pressure plate is provided with a plurality of holes 39 for receiving the tops of the rods springs 33, the tops of which engage the bottom of the pressure plate 21 and the bottoms of which engage the discs 32 which are attached to the bottom ends of the rods 3|. The discs 32 rest upon the top surfaces of the piezoelectric crystal wafers 23 that are being lapped and apply the spring pressure uniformly to said crystal blanks. The holes in the pressure plate are large enough so that they do not touch the shafts of the rods 35 otherwise these rods would be pushed out of their free positions and the It will be appreciated that modifications of this apparatus may be made without departing from the spirit and scope thereof and therefore it is not desired to limit this invention to the exact embodiment illustrated and described except insofar as this may be defined by the Jclaims.

I claim: 1. Apparatus for lapping piezoelectric crystal blanks to relatively thin dimensions comprising a lap plate, a work holder having a face thereof in engagement with the lapping face of the lap plate, means operably connected to said work holder for moving said work holder and said lap plate with respect to each other, said work holder having a hole formed therein to "receivea piezoelectric crystal blank having a pair of ma; or faces the diameters of which are large compared to the thickness of the blank, said work holder being adapted to guide said crystal blank against the lapping surface of said -lap plate, and a weight member freely positioned in said hole for applying pressure to said .piezoelectric crystal blank, the face of said weight member engaging a major face of said crystal blank being convex so that the major face of said crystal blank being lapped is lapped concave.

2. Apparatus for lapping piezoelectric crystal blanks to relatively thin dimensions comprising a lap plate, a work holder having a face thereof in engagement with the lapping face of the lap plate, means operably connected to said work holder for moving said work holder and said 'lap plate with respect to each other, said work holder having a hole formed therein to receive These rods 3! are encircled by helical a piezoelectric crystal blank having a pair of ma or faces the diameters of which are large compared to the thickness of the blank, said work holder being adapted to guide said crystal blank against the lapping surface of said lap plate, and a weight member freely positioned in said hole for applying pressure to said piezoelectric crystal blank, the face of said weight member engaging a major face of said crystal blank being concave so that the major face of said crystal blank being lapped is lapped convex.

3. Apparatus for lapping piezoelectric crystal blanks to relatively thin dimensions comprising a lap plate, a work holder having a face thereof in engagement with the lapping face of the lap plate, means operably connected to said work holder for moving said work holder and said lap plate with respect to each other, said work holder having a hole formed therein to receive a piezoelectric crystal blank having a pair of major faces the diameters of which are large compared to the thickness of the blank, said work holder being adapted to guide said crystal blank over the lapping surface of said lap plate, a weight member positioned in said hole to ride on said piezoelectric crystal blank for applying pressure to said piezoelectric crystal blank uniformly over one of the major faces of said blank to secure uniform lapping thereof, a resilient member positioned against said weight member and a member adjustably attached to said work holder for applying pressure to said weight member through said resilient member so that the rate of lapping of said crystal blank may be controlled to prevent fracture thereof, the face of said weight member engaging a major face of said crystal blank being convex so that the major face of said crystal blank being lapped is lapped concave. I

4. Apparatus for lapping piezoelectric crystal blanks to relatively thin dimensions comprising a lap plate, a work holder having a face thereof in engagement with the lappin face of the lap plate, means operably connected to said work holder for moving said work holder and said lap plate with respectto each other, said work holder having a hole formed therein to receive a piezoelectric crystal blank having a pair of major faces the diameters of which arelarge compared to the thickness of the blank,-said work holder being adapted to guide said crystal blank over the lapping surface of said lapplate, a weight member positioned in said holeto ride on said piezoelectric crystal blank for applying pressure to said piezoelectric crystal blank'uniformly over one of the major facesof said blank to secure unifrom lappin thereof, a resilient member positioned against said weight member and a member adjustably attached to said work holder, for applying pressure to said weight 'mernber through said resilient member, so that the rate of lapping of said crystal blank may ibe controlled to prevent fracture thereof, the

face of said weight member engaging a major face of said crystal blank being concave so that the major face of said crystal blank being lapped is lapped convex.

atively'connected to said lap plate for rotating said lap plate, a piezoelectric crystal guiding member having a plurality of holes for-med therein for receiving piezoelectric crystals of 9 thin wafer-like configuration, said holes having diameters corresponding substantially to the diameters of said piezoelectric crystals, means operatively connected to said piezoelectric crystal guiding member for moving said guiding member on the top of said lap plate, weight members fitting into said holes for uniformly pressing a surface of each of said piezoelectric crystals against said lap plate without distorting the shapes of said piezoelectric crystals, insulation means for electrically insulating said weight members from said guiding member, and frequency measuring apparatus connected to said insulated weight members and to said, lap plate for indicating the frequency of said crystal blanks;

JOHN M. WOLFSKIIL.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number 10 UNITED STATES PATENTS Name Date Bacon June 1, 1880 Macbeth M Apr. 29, 1890 Setter July 12, 1904 Tunich July 27, 1915 Lowe Apr. 9, 1918 Straubel May 1, 1934 Strieby Dec. 10, 1935 Bailey Feb. 1, 1944 Mac Connell July 11, 1944 Fruth July 16, 1946 Primus Apr. 15, 1947 Ramsay July 1, 1947 Bach Jan. 27, 1948 FOREIGN PATENTS Country Date Great Britain Jan. 2, 1939 

